This invention relates generally to the production of extreme ultraviolet and soft x-rays with an electric discharge source for projection lithography.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths. Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only when serving several steppers.
Another source is the laser plasma source (LPS), which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. LPS is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). The plasma is produced by a high-power, pulsed laser that is focused on a metal surface or in a gas jet. (See, Kubiak et al., U.S. Pat. No. 5,577,092 for a LPS design.)
Discharge plasma sources have been proposed for photolithography. Capillary discharge sources have the potential advantages that they can be simpler in design than both synchrotrons and LPS""s, and that they are far more cost effective. Klosner et al., xe2x80x9cIntense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography,xe2x80x9d Opt. Lett. 22, 34 (1997), reported an intense lithium discharge plasma source created within a lithium hydride (LiH) capillary in which doubly ionized lithium is the radiating species. The source generated narrow-band EUV emission at 13.5 nm from the 2-1 transition in the hydrogen-like lithium ions. However, the source suffered from a short lifetime (approximately 25-50 shots) owing to breakage of the LiH capillary.
Another source is the pulsed capillary discharge source described in Silfvast, U.S. Pat. No. 5,499,282, which promised to be significantly less expensive and far more efficient than the laser plasma source. However, the discharge source also ejects debris that is eroded from the capillary bore and electrodes. An improved version of the capillary discharge source covering operating conditions for the pulsed capillary discharge lamp that purportedly mitigated against capillary bore erosion is described in Silfvast, U.S. Pat. No. 6,031,241.
Debris generation remains one of the most significant impediment to the successful development of the capillary plasma discharge sources in photolithography. Debris generated by the capillary tends to coat optics used to collect the EUV light which severely affects their EUV reflectance. Ultimately, this will reduce their efficiency to a point where they must to be replaced more often than is economically feasible. The art is in search of capillary plasma discharge sources that do not generate significant amounts of debris.
The present invention is based in part on the demonstration that constructing the capillary bore of an extreme ultraviolet electric plasma discharge with boron nitride can significantly reduce the amount of debris generated. A corollary feature is that the flux of radiation produced is also increased. Applications for the inventive light source include, for example, commercial EUV lithography, microscopy, metrology, and mask inspection.
In one embodiment, the invention is directed to an extreme ultraviolet and soft x-ray radiation electric discharge plasma source that includes:
(a) a body made of boron nitride that defines a capillary bore that has a proximal end and a distal end;
(b) a first electrode defining a channel that has an inlet that is connected to a source of gas and an outlet end that is in communication with the distal end of the capillary bore;
(c) a second electrode positioned to receive radiation emitted from the proximal end of the capillary bore and having an opening through which radiation is emitted; and
(d) a source of electric potential that is connected across the first and second electrodes.
In another embodiment, the invention is directed to a method of producing extreme ultra-violet and soft x-ray radiation that includes the steps of:
(a) providing an electric discharge plasma source that includes:
(i) a body made of boron nitride that defines a capillary bore that has a proximal end and a distal end;
(ii) a first electrode defining a channel that has an inlet that is connected to a source of gas and an outlet end that is in communication with the distal end of the capillary bore;
(iii) a second electrode that is positioned adjacent to the proximal end of the capillary bore and defining an orifice;
(iv) a source of electric potential that is connected across the first and second electrodes; and
(v) a second housing that defines a vacuum chamber that is in communication with the orifice;
(b) introducing gas from the source of gas into the channel of the first electrode and into the capillary bore; and
(c) causing an electric discharge in the capillary bore sufficient to create a plasma within the capillary bore thereby producing radiation of a selected wavelength.
Preferred boron nitrides for the housing are in the form of pyrolytic boron nitride, compression annealed pyrolytic boron nitride, and cubic boron nitride.
Capillary bore materials used in previous electrical discharge sources have suffered from significant bore erosion and debris generation at all operating conditions of interest for EUV photolithography. The intense plasma generated in the capillary bore tends to heat the capillary walls above the melting temperatures of most materials. Depending on the material used, this causes the bore surface either to vaporize directly or to repeatedly melt and freeze. This cyclic melting and freezing changes the material""s crystalline structure. Moreover, significant stresses are introduced near the surface of the capillary by intense thermal gradients generated during the discharge cycle. The combination of these stresses and the change in the materials structure cause chunks of material to break off from the surface. Both the vaporization and fracturing tend to increase the capillary bore diameter and generate unwanted debris. This debris streaming in from the walls also tends to cool the plasma. This cooling effect is thought to be responsible for an abrupt decline in EUV emission observed during the discharge cycle. Because boron nitride, e.g., pyrolytic boron nitride, has a higher melting temperature and lower vapor pressure and is extremely resistant to fracture under stress, less bore material is expected to be introduced into the plasma resulting in decreased bore erosion and debris generation and increased EUV flux.
In one preferred embodiment, the proximal end of the capillary bore is connected to the nozzle of the second electrode wherein the nozzle has a conical inner surface which radially expands in an outward direction and the conical inner surface has an inlet having a diameter that is larger than the diameter of the proximal end of the capillary bore and the distance from the center of the capillary bore. The nature of the plasma/material interaction in the capillary bore is such that a capillary material with the following characteristics at elevated temperature are required: low vapor pressure, high mechanical strength, low thermal expansion, high thermal conductivity and high dielectric strength. Pyrolytic, compression annealed pyrolytic, and cubic are forms of boron nitride that have been identified as possessing these properties.
In another preferred embodiment, the housing comprises an inner core made of boron nitride that has a capillary bore and an outer member, positioned around the inner core and that is made of a more thermally conductive dielectric material.